By Adam J. Pearson
Note: this is an old article. For a more recent one that discusses the same topic, feel free to check out “The Integral ‘Now:” Towards an Integration of Einstein’s Relative Simultaneity and Eckhart Tolle’s Eternal Present”
What is the view of time that Albert Einstein presents to us in special relativity? Einstein tells us that there is no separate ‘time’ or ‘space.’ ‘Time’ and ‘space’ cannot be separated; they are a united whole. The past, the present, and the future all exist at once in a single four-dimensional spacetime continuum. This continuum is real, is reality. We, distinct observers, move through this continuum at different velocities and slice out different ‘nows’ from it.
Rejecting Isaac Newton’s view of “absolute time,” Einstein explains that we cannot speak of a single absolute “now” common to all observers in all frames of references. As special relativity states, people moving in relative motion at differing velocities experience different ‘nows.’ If we are moving substantially faster than another person and look at a clock in their reference frame, we will see time dilation – a delay of time relative to our own point of view. There is no single, absolute Now for Enstein; instead, there are as many ‘nows’ as there are observers in relative motion. Or, to state the matter different, there are as many ‘nows’ as there are frames of reference from which to see, and there are as many of those as there are velocities at which an object can travel (a nearly infinite number).
What does this mean for the old Newtonian view that we can speak of ‘simultaneous events’? It means that we must, in the colourful words of Hume, “commit it to the flames.” In Einstein’s universe–in our universe as seen by special relativity, or as Einstein called it, invariant theory because the spacetime interval is invariant for all observers–there are no simultaneous events across different reference frames. Because two observers in relative motion cannot agree on a single ‘now,’ we cannot speak of two events occurring ‘simultaneously’ in two different frames of reference. There is no absolute standard by which we could determine simultaneity; all we have to work with are the perspectives of particular observers either at rest relative to some reference frame or moving at different relative velocities to one another. The Newtonian Now has been shattered into the Einsteinian nows and with this shattering, simultaneity across reference frames has itself ceased to hold meaning.
Moreover, while special relativity slices the Newtonian Now into relative nows based on relative motion, general relativity further warps it by introducing a new model of gravity. According to general relativity, massive objects like stars and planets warp or curve the fabric of spacetime around them such that objects that come into their gravitational field move, not in straight lines, but along curved lines called geodesics. General relativity’s claim is striking; massive objects warp the very dimension of time in their immediate vicinity. A key implication of this reality is that clocks within gravitational wells, within the gravitational fields of large objects, move more slowly than clocks outside of them. There is, in other words, not only time-dilation based on relative velocity, but also gravitational time dilation. Gravitational fields slow down the observed passage of time within their sphere of influence. In general relativity, in other words, the time dimension is not a smooth-flowing arrow; it’s more like a spread-out blanket tugged and warped here and there by the distribution of massive objects within it.
In short, special and general relativity, once properly understood, require us to throw out the old Newtonian notions of a single, absolute Now and an absolute time that is totally separate from space. In their place, what we find is a magnificent universe in which space and time are inseparately interconnected into a single 4-dimensional spacetime. We find a universe in which observers in relative motion experience time differently and cannot agree on simultaneous events; two events that are simultaneous in one reference frame are not simultaneous in another (see Minkowskian spacetime diagrams for a visual picture of this concept). We find a realm in which Newton’s absolute Now has been shattered into countless nows relative to all of the observers that move through spacetime. We find a world in which even gravity shapes the observed passage of time. In closing, we need not mourn the death of the old Newtonian view, for in its place, Einstein has given us a much deeper, much richer view of the complexity and powerful relativity of our amazing universe.